Hungate, Robert E. Department of Bacteriology, University of California, Davis, California.
Halvorson, Harlyn O. Department of Biochemistry, University of Minnesota, Minneapolis, Minnesota.
Hutchison, Keith Department of Biology, Brandeis University, Waltham, Massachusetts.
Orrego, Cristian Department of Biology, Brandeis University, Waltham, Massachusetts.
Last reviewed:March 2019
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Extremely small, relatively simple microorganisms possessing the prokaryotic type of cell construction. Bacteria are very small prokaryotic microorganisms, usually measuring 0.3 to 2.0 micrometers in diameter (Fig. 1). They are found almost everywhere, being abundant in soil, water, and the alimentary tracts of animals. Each kind of bacterium is fitted physiologically to survive in one of the innumerable habitats created by various combinations of space, food, moisture, light, air, temperature, inhibitory substances, and accompanying organisms. Dried bacteria, but often still living, can be carried into the air. One of the few locations in which bacteria are not usually found is within the cells of other healthy organisms, although even this is subject to exceptions; for example, there are many bacteria that do live intracellularly in a number of eukaryotic organisms. See also: Bacterial growth; Bacterial physiology and metabolism; Bacteriology; Microbiology; Prokaryote
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